Huntington?s disease (HD) is an inherited adult-onset neurodegenerative disorder caused by an abnormal expansion of CAG codons in the huntingtin (HTT) gene. HD is characterized by the aggregation of mutant HTT (mHTT) protein and selective degeneration of striatal medium spiny neurons (MSNs). Modeling HD using patient-derived neurons has been challenging mainly due to the lack of experimental approaches to obtain adult neurons from HD patients. Our previous work demonstrated that human MSNs could be generated with high efficiency and specificity from adult skin fibroblasts through direct cell fate conversion (reprogramming) using microRNAs and transcription factors. Importantly, the converted human MSNs resembled the neurons of human adults, an important feature for modeling late-onset diseases. However, the utility of directly converted MSNs as a cellular model of adult-onset HD remained to be determined. Recently, our preliminary work demonstrated that MSNs could be generated from directly converting fibroblasts of HD patients (HD-MSNs), and the resulting HD-MSNs manifested key hallmarks of HD pathology such as mHTT aggregates, DNA damage, and spontaneous cell death in culture. In the current grant, we propose to use HD-MSNs as a cellular model of HD and define genetic factors that alleviate the neuronal death of HD-MSNs. In Aim 1, we focus on SP9, a transcription factor that we found to be significantly downregulated in HD-MSNs in comparison to control MSNs from healthy individuals (Ctrl-MSNs). Interestingly, SP9 has been reported to be required for the maintenance and survival of MSNs, and we discovered that enforcing SP9 expression in HD-MSNs protected the cells from spontaneous cell death. To define the neuroprotective role of SP9 in HD-MSNs, we will identify direct target genes of SP9 and reveal genes integral to SP9?s function to promote HD-MSN survival. In Aim 2, we will investigate the function a primate-specific microRNA, miR-663b as a neuroprotective miRNA in HD- MSNs. Our preliminary work indicated that miR-663b protected MSNs from oxidative stress-induced neurodegeneration. Given the link between oxidative cellular stress and neurodegeneration in HD-MSNs, we will test if increasing the miR-663b level in HD-MSNs would confer a neuroprotection and identify direct target genes of miR-663b to delineate the function of miR-663b in HD-MSNs. In Aim 3, we will identify genetic pathways responsible for differential vulnerability to neuronal death between MSNs at different stages of disease progression. We found that HD-MSNs generated from fibroblasts sampled before the onset of clinical symptoms (pre-HD-MSNs) displayed significantly lower degrees of DNA damage and cell death in comparison to HD-MSNs derived after the onset of clinical symptoms. We will conduct transcriptome analysis to identify differentially expressed genes between pre-HD-MSNs and symptomatic HD-MSNs and identify differentially expressed genes responsible for the differential vulnerability to neuronal death. Overall, results from the current proposal will provide insights to neuronal death in HD using patient-derived neurons.